Organic-based electronic devices, in which the active semiconducting material is an organic compound (e.g., pentacene) or conjugated polymer (e.g., poly(3-alkylthiophene)), are attractive for their relatively inexpensive, low-temperature processability. However, such organic-based devices generally have limited application due to the low field effect mobilities (μ) realized in such devices and their poor reliability (e.g., reproducibility, variable threshold voltage, air stability, and processability). Different organic materials have been screened in traditional organic thin film transistors (TFTs), yet the mobilities in such materials are generally not much greater than about 1 cm2/Vsec (C. Dimitrakopoulos et al., “Organic Thin Film Transistors for Large Area Electronics,” Adv. Mater., 2002, 14(2), 99-117). While organic TFTs with field effect mobilities of 15 cm2/Vsec have been reported with pentacene devices on treated silicon via evaporation, such results have not been reproducible (T. Kelly et al., ACS ProSpective Meeting, Thin-Film Electronics: Materials, Devices, and Applications, Jan. 25-28, 2004, Miami, Fla.). Single crystals of organic materials have been shown to have mobilities as high as 10-15 cm2/Vsec, but they are difficult to produce and still do not approach the mobility of 100 cm2/Vsec anticipated to be required for high-performance devices and circuits (V. Sundar et al., “Elastomeric Transistor Stamps: Reversible Probing of Charge Transport in Organic Crystals,” Science, 2004, 303, 1644-1647). Thus, high-performance field effect transistors (FETs) on plastic cannot be achieved with traditional organic materials, or, for that matter, with silicon, since in the latter case processing methods are limited to amorphous silicon.
It would be highly desirable to be able to manufacture high mobility TFTs with low-temperature, solution-based processing techniques that would allow low cost, high-performance devices for large area electronics. Indeed, such a processing method has been a long-sought after goal as higher mobilities would allow faster switching in high-end displays and permit logic applications (C. Dimitrakopoulos et al., “Organic Thin Film Transistors for Large Area Electronics,” Adv. Mater., 2002, 14(2), 99-117). Hence, the problem to be overcome is three-fold: limited processing capability, poor performance, and poor reliability of traditional organic TFTs.
There has been considerable effort to overcome the above-described problems by replacing the semiconducting material in such TFTs with a carbon nanotube (CNT) network (Baughman et al., Carbon Nanotubes—the Route Toward Applications,” Science, 2002, 297, 787-792; Dai, “Carbon Nanotubes: Synthesis, Integration, and Properties,” Acc. Chem. Res., 2002, 35, 1035-1044). While CNT transistors based on the use of a single CNT per channel are not currently commercializable due to expensive, unreliable, and uncontrollable processes, CNT transistors based on CNT networks have been prepared by growing CNT networks on silicon substrates at high temperatures, temperatures that are not compatible with plastic substrates. Transport properties of such single-wall carbon nanotube (SWNT) network transistors have been reported as having mobilities of 10 cm2/Vsec and Ion/Ioff of 105. At higher SWNT densities, mobilities of 100 cm2/Vsec are obtained, but with a high off current (Ioff) (Snow et al., “Random Networks of Carbon Nanotubes as an Electronic Material,” Appl. Phys. Lett., 2003, 82(13), 2145-2147). Furthermore, such in situ growth of CNTs directly on substrates provides an uncontrollable mixture of semiconducting and metallic tubes such that on/off ratios of such devices are poor (Xiao et al., “High-Mobility Thin-Film Transistors Based on Aligned Carbon Nanotubes,” Appl. Phys. Lett., 2003, 83, 150-152). Such in situ growth processes also tend to be low yield processes.
Martel et al. (Appl. Phys. Lett., 1998, 73, 2447) prepared single-tube devices by dispersing a dilute suspension of CNTs onto a substrate and then patterned electrodes on the surface comprising the CNTs. However, this process is not practical, as described therein, and the metallic CNTs still present can lead to short-circuited devices.
The fabrication of TFTs based on single-wall carbon nanotube (SWNT) networks has been accomplished on silicon substrates from which they were then transferred to plastic substrates (Bradley et al., “Flexible Nanotube Electronics,” Nano Lett., 2003, 3(10), 1353-1355). Such a transfer process is not practical or cost-effective, however, and growing tubes (individual or network) on plastic is not possible due to the high temperatures typically required.
DNA-streptavidin complexes have been used to assemble templated CNT FETs using single, isolated semiconducting CNTs (Keren et al., “DNA-Templated Carbon Nanotube Field-Effect Transistor,” Science, 2003, 302, 1380-1382). However, as already mentioned, such single CNT devices are not practical and such methods still require isolation of semiconducting CNTs.
Efforts to overcome processing limitations of in situ CNT growth for FET devices have led some to fabricate silicon-based nanowires on plastic substrates (McAlpine et al., “Nanoimprint Lithography for Hybrid Plastic Electronics,” Nano Lett., 2003, 3(4), 443-445; Duan et al., “High-Performance Thin-Film Transistors Using Semiconductor Nanowires and Nanoribbons,” Nature, 2003, 425, 274-278; McAlpine et al., “High-Performance Nanowire Electronics and Photonics on Glass and Plastic Substrates,” Nano Lett., 2003, 3(11), 1531-1535). Such nanowires are limited, as they are typically produced in very low yield and readily oxidize in air. Furthermore, inorganic nanowires suffer from trapped states on the nanowire surface and difficulties in doping.
Several recent publications have described processes to separate semiconducting SWNTs from metallic SWNTs (D. Chattopadhyay et al., “A Route for Bulk Separation of Semiconducting from Metallic Single-Wall Carbon Nanotubes,” J. Am. Chem. Soc., 2003, 125, 3370; M. Zheng et al., “Structure-Based Carbon Nanotube Sorting by Sequence-Dependent DNA Assembly,” Science, 2003, 302, 1545-1548; Weisman, “Four Degrees of Separation,” Nat. Mater., 2003, 2, 569-570), yet no one has used these processes to fabricate TFT based on solution-cast SWNT networks that are enriched with semiconducting SWNTs.
Selective chemistry to render the metallic SWNTs non-conducting has been developed. Such chemistry selectively reacts metallic SWNTs in the presence of semiconducting SWNTs. Such chemistry disrupts the conjugation of the metallic SWNTs and effectively destroys their metallic character (M. Strano et al., Science, 2003, 301, 1519). Recently, this approach has been used to fabricate FETs comprising CNTs grown in situ on a device platform (L. An et al., “A Simple Chemical Route to Selectively Eliminate Metallic Carbon Nanotubes in Nanotube Network Devices,” J. Am. Chem. Soc., 2004, 126(34), 10520-10521), but such processing still requires high temperatures to generate the CNTs and leaves chemically-destroyed metallic CNTs in the device and this chemistry is acknowledged by the authors to not be completely selective.
In light of the above, a method to inexpensively manufacture FETs, and TFTs in particular, at low temperatures using both solution-based processing and CNTs to provide high field effect mobilities would be highly desirable, as it would permit such devices to be fabricated with plastic substrates. Such resulting low-cost devices would allow their incorporation into articles of manufacture heretofore economically unrealizable.